President Obama stands next to acting Director of Office of Management and Budget Jeffrey Zients in the Rose Garden at the White House in Washington earlier this year. Mr. Zients is now in charge of overseeing the effort to fix the Obamacare website.

But in an apparent reply to administration outsiders who have said the software should be rewritten from scratch, he asserted that “Healthcare.gov is fixable.”

Some technology experts, while not necessarily calling for a ground-up rewrite, say the fixes could easily end up stretching beyond the end-of-November target.

Last week, at a hearing held by the House Energy and Commerce Committee, representatives of some of the firms the government hired to build HealthCare.gov said the process of testing such a complex system before launch typically might run for months. The site didn’t get that kind of testing. Now the rush is on to fix and test the fixes within weeks.

The stakes for Americans and for Mr. Obama are high. The timeline for a fix to the HealthCare.gov website is important to those trying to set their health insurance plans for next year.

Dec. 15 is when people need to be signed up in order to have coverage begin on Jan. 1. Even if the website works well by Dec. 1 for virtually all users that would leave two weeks for a potentially large flow of signups.

The timing could also help influence whether the president’s health-care reform law goes down as a success or failure. A key premise of the law’s “individual mandate” (to carry insurance or pay a tax penalty) is that millions more Americans will obtain coverage – including young and healthy customers as well as ones who use more medical services.

A crash-prone signup process could deter the 20- and 30-somethings who are least desperate for insurance, but whose presence is vital for Obamacare’s budget math.

By all accounts, a core test will be whether the tech efforts can overcome problems that have resulted in a garbled or inaccurate flow of data from the site to health insurance firms. Insurance companies have reported widespread problems verifying enrollment information passed along via HealthCare.gov.

“It is at the top of the punch list” of fixes, says Mr. Zients, who is set to become the president’s chief economic adviser in January.

Since its launch, the website has run anything but smoothly. First, the system broke down as users began setting up individual accounts for enrollment. In addition, the site’s architecture steered everyone to open an account, even if they just wanted to “window shop” for possible health plans.

The site soon added a window-shop feature, which itself drew criticism for giving misleading information. Meanwhile the snarled flow of information to insurers was emerging.

The site got a cosmetic change over the weekend: No longer is the home screen dominated by a smiling woman’s face. Now visitors see headers about various enrollment options.

More fundamentally, control of the troubleshooting has shifted. Zients said that one of the firms that helped build HealthCare.gov, Quality Software Services Inc., or QSSI, would serve in a general contractor role as the bugs are removed. That shifts responsibility away from the federal Centers for Medicare and Medicaid Services (CMS), which managed various contractors as the site was developed.

Even as the techies race to patch the software, debate is likely to simmer about whether the Obama administration or Congress should adjust Affordable Care Act deadlines because of the delays.

Julie Bataille, a spokeswoman for CMS, said that already the website is allowing some Americans to enroll and the site should work “better and better each week” so that “people will be able to apply by Dec. 15 in order to get coverage Jan. 1.”

For this to play out as the Obama administration hopes, some teams of code writers will have to be both busy and successful in coming days.

As Zients put it, “we need these folks to have their heads down, executing, spotting any additional issues and getting things done off that checklist.”